Class 12 Physics Current Electricity Notes

Class 12 Physics Current Electricity Notes

3.1 Introduction

Current electricity deals with the flow of electric charges through conductors. It explains how electric current is produced, how it flows in circuits, and how electrical devices use energy. These concepts form the basis of household and industrial electrical systems.

3.2 Electric Current

Electric current is defined as the rate of flow of electric charge through a conductor.I=QtI = \frac{Q}{t}I=tQ​

  • SI unit: ampere (A)
  • Direction of conventional current is from positive to negative terminal.
  • In reality, electrons flow in the opposite direction.

3.3 Electric Currents in Conductors

In metallic conductors:

  • Free electrons are responsible for current flow.
  • In the absence of an electric field, electrons move randomly.
  • When an electric field is applied, electrons acquire a drift motion, producing electric current.

3.4 Ohm’s Law

Ohm’s law relates voltage, current, and resistance.

Statement:
At constant temperature, the current through a conductor is directly proportional to the potential difference across its ends.V=IRV = IRV=IR

where
R = resistance of the conductor

  • SI unit of resistance: ohm (Ω)

3.5 Drift of Electrons and the Origin of Resistivity

Drift velocity is the average velocity with which free electrons move under an applied electric field.

  • Drift velocity is very small.
  • Resistivity arises due to collisions of electrons with atoms and ions of the conductor.
  • Greater collisions → higher resistivity.

3.6 Limitations of Ohm’s Law

Ohm’s law is not valid for:

  • Semiconductor devices (diodes, transistors)
  • Electrolytes
  • Vacuum tubes
  • Conductors at very high temperatures

In these cases, the relationship between V and I is non-linear.

3.7 Resistivity of Various Materials

Resistivity depends on the nature of the material.

  • Low resistivity: Copper, silver
  • High resistivity: Nichrome, manganin
  • Very high resistivity: Insulators

Resistivity is independent of shape and size.

3.8 Temperature Dependence of Resistivity

Resistivity changes with temperature.

  • For metals, resistivity increases with temperature.
  • For semiconductors, resistivity decreases with temperature.

This property is used in temperature sensors.

3.9 Electrical Energy and Power

Electrical energy is the energy consumed by an electrical device.E=VItE = VItE=VIt

Electrical power is the rate of consumption of electrical energy.P=VI=I2R=V2RP = VI = I^2R = \frac{V^2}{R}P=VI=I2R=RV2​

  • SI unit of power: watt (W)

3.10 Cells, EMF and Internal Resistance

A cell converts chemical energy into electrical energy.

  • EMF (ε): Potential difference between terminals when no current flows.
  • Internal resistance (r) opposes the flow of current inside the cell.

V=ε−IrV = \varepsilon – IrV=ε−Ir

3.11 Cells in Series and in Parallel

Cells in Series

  • EMFs add up.
  • Used when high voltage is required.

Cells in Parallel

  • Internal resistance decreases.
  • Used when large current is required.

3.12 Kirchhoff’s Rules

Kirchhoff’s rules are used to analyze complex circuits.

1. Junction Rule

The algebraic sum of currents at a junction is zero.

2. Loop Rule

The algebraic sum of potential differences in a closed loop is zero.

3.13 Wheatstone Bridge

Wheatstone bridge is used to accurately measure unknown resistance.

  • It works on the principle of null deflection.
  • At balance condition:

R1R2=R3R4\frac{R_1}{R_2} = \frac{R_3}{R_4}R2​R1​​=R4​R3​​

It is widely used in laboratory measurements.